Heights of solar tracers observed at 8 mm and an interpretation of their radiation

¹ Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kačićeva 26, 10000 Zagreb, Croatia e-mail: romanb@geof.hr
² Zagreb Astronomical Observatory, Opatička 22, 10000 Zagreb, Croatia e-mail: ivan.romstajn@po.t-com.hr; drosa@zvjezdarnica.hr
³ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany e-mail: hw@kis.uni-freiburg.de
⁴ Institute of Astronomy, ETH-Hönggerberg, HIT J 23.1, 8093 Zürich, Switzerland e-mail: benz@astro.phys.ethz.ch
⁵ Observatorium Kanzelhöhe/IGAM, Institut für Physik, Universität Graz, Universitätsplatz 5, 8010 Graz, Austria e-mail: manuela.temmer@uni-graz.at

Received: 31 May 2008
Accepted: 18 September 2008

Abstract

Context. At the wavelength of 8 mm, emissive features (high brightness-temperatrue regions, HTRs) and absorptive features (low brightness-temperature regions, LTRs) can be traced for the determining the solar rotation. From earlier studies it is known that about two thirds of LTRs are associated with Hα filaments.

Aims. Thermal bremsstrahlung and gyromagnetic (cyclotron) radiation mechanism can be important for explaining the observed phenomena, so we determine the heights of solar structures and interpret their radiation mechanism(s).

Methods. We use the method of simultaneous determination of the solar synodic rotation velocity and the height of tracers. The rotation velocities were determined by the linear least-square fit of their central meridian distance as a function of time. We used a procedure for calculating the brightness temperature for a given wavelength and model atmosphere, which integrates the radiative transfer equation for the thermal bremsstrahlung.

Results. The mean value of the low brightness-temperature regions' heights is about 45 600 km. This height was used as input for constructing prominence and coronal condensation models, which, when assuming thermal bremsstrahlung as the radiation mechanism, yield a decrease in the brightness temperature of 2–14%, in agreement with observations. If the same radiation mechanism is considered, the models of the solar corona above active regions give an increase in the brightness temperature of 5–19%, also in agreement with observations. In this case an indirect indication (from the rotational analysis) that the HTRs are located higher in the solar atmosphere than the LTRs was taken into account.

Conclusions. The method for simultaneously determining the solar synodic rotation velocity and the height of tracers could have only been properly applied on LTRs, since a homogeneous distribution over latitudes and central meridian distances of a large enough data set is necessary. Thermal bremsstrahlung can explain both the LTR (prominences and coronal condensations) and HTR (ordinary active regions) phenomena observed at 8 mm. At this wavelength, thermal gyromagnetic emission is almost surely excluded as a possible radiation mechanism.